The time it takes to refuel a nuclear power plant is very costly because of the replacement power which has to be purchased during such time. It is common in the operation of nuclear reactors to provide a flange cover to create a full and complete seal between the nuclear power plant's primary containment area and the fuel transfer canal or tube that leads to the fuel storage pool. The fuel storage pool generally contains water and is sunk into the earth for the handling and storage of irradiated reactor components such as control rods or fuel assemblies. In the refueling of a reactor, irradiated or spent fuel assemblies are transferred from the reactor pit to the fuel storage pool for temporary storage. The transfer is accomplished under water to protect operators from the effect of radiation.
When refueling the nuclear power plant it is necessary for operating personnel enter an area that is posted either a high radiation or locked high radiation area or even an airborne contamination area to remove the flange cover on the containment side of the fuel transfer tube. After the refueling is completed the reactor cavity is drained and the operating personnel reenter the containment area to reinstall the flange cover. The flange cover is designed to be structurally sound and to maintain its seal under several design conditions including: normal pressure on the containment side of the flange cover and approximately 15 pounds per square inch (psi) water head on the fuel transfer tube or canal side of the flange cover; accident condition of 45 psi on the containment side of the flange cover and either 15 psi of water head or essentially air and atmospheric pressure on the fuel transfer tube side of the flange cover; and seismic forces simulating the additional pressure burden from an earthquake.
The flange cover generates a seal with two O-rings that are positioned in two spaced grooves machined into the sealing side of the flange. The flange cover is attached to a flat faced weld neck flange on the fuel transfer tube by a number of bolts which must be removed and reinstalled for each refueling event. For example, a typical flange cover might be a 60 inch nominal pipe size flange that is mated to a 60 inch nominal flat faced weld neck flange on the fuel transfer tube. The flange cover is bolted in place using bolts or studs, washers and nuts and a full set might have 44 bolts, nuts and washers. The reinstallation is particularly time consuming and exposes workers to additional radiation dose because the bolts must be tightened to a prescribed torque in several passes in a specified sequence to achieve a full compression on the O-rings seals to assure a leak proof seal.
The manual installation of the flange cover on the fuel transfer tube flange involves two to three operators dressed in contamination protective clothing including filtered air hood or respiratory devices and about an hour to remove the flange cover and about two hours to reinstall it. The containment area is hot and contains sufficient radiologic contamination to expose the operators to significant radiation. These operations are performed in an environment that could have as high as three rem per hour after each refueling. It is known that the longer an operator stays in proximity to radioactive material the greater his/her radiation exposure and therefore it is preferable to minimize any such exposure below the permissible radiation levels specified by government regulations. It is desirable therefore to be able to remove and replace the flange cover so as to reduce both the refueling time and operator radiation exposure.
There have been numerous approaches with the aim of reducing operator exposure to radiation in the removal and reinstallation of the flange cover including reducing the number of bolts to approximately six or eight bolts to bolt the flange cover to the fixed flange on the fuel transfer tube. Although the time an operator is exposed to radiation is somewhat reduced significant exposure still remains.
So called quick release hatches or covers generally have complex latching mechanisms including a plurality of radially outwardly and inwardly slidable locking bolts that must align with and slide into latch receiving openings on a locking ring that is added to the flange weldment on the fuel transfer tube, and therefore are not suitable for use on existing fuel transfer tube flange weldments.
The flange cover also typically includes a test port extending from the exterior containment side of the flange cover to a space between the two O-rings. Air or gas is introduced at a specific pressure and the leakage across the O-ring seals is measured using the appropriate measuring instruments to insure the leakage rate is below 25 standard cubic centimeters per minute. The leakage testing and measuring procedure further extends the radiation exposure time of an operator.
It would be desirable therefore to provide a flange cover that can be remotely installed and/or uninstalled from the fuel transfer tube flange without modification to the existing fuel transfer tube flange. It would also be desirable to remotely test and measure the leakage rate across the O-rings when the flange cover is sealingly attached to the fuel transfer tube flange.